Discretizing an Aircraft Surface into P-Static Zones

The present disclosure relates generally to triboelectric charging of aircraft surfaces, and in particular, to modeling zones of the aircraft surface based on particle impingement and precipitation charging profiles.

Precipitation static (p-static) refers to electrostatic charging of aircraft surfaces due to collision with particles during flight. Impingement of particles transfers charge at the point of impact on the aircraft exterior, resulting in electrostatic charge accumulation. This can cause interference with aircraft navigation and communication systems by broadband discharges. Since aircraft cannot realistically avoid operating in situations where p-static charging occurs, manufacturers design and certify aircraft in terms of p-static performance by envisioning “worst case scenario” situations, despite the fact p-static charging can be non-uniform. This leads to excessive expense and overly conservative designs.

A method is presented for discretizing an aircraft surface into p-static zones. The method comprises generating a particle impingement model for an aircraft surface using computational fluid dynamics. The particle impingement model is discretized to generate two or more p-static zones. P-static design guidelines are established on a per p-static zone basis. Unbonded conductive materials and/or dielectric materials are then applied to the aircraft surface based on the p-static design guidelines.

This Summary is provided in order to introduce in simplified form a selection of concepts that are further described in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any disadvantages noted in any part of this disclosure.

Aircraft flying through suspended atmospheric particles, including dust, ice crystals, and precipitation, are subject to accumulating electric charge through the triboelectric effect and charged particle exchange. Surface charge accumulates on the aircraft surface and dissipates into structure. Additional charging of aircraft can be observed when flying through strong electric fields. If not properly dissipated, this charge can accumulate on different locations and create localized arcing, resulting in high localized currents and electromagnetic fields. These fields can couple to aircraft communication and navigation systems. This aircraft charging and antenna coupling phenomena is commonly referred to as p-static interference. Federal Aviation Administration (FAA) regulations govern normal aircraft operation in severe p-static environments when designing aircraft.

General guidance for p-static design includes electrical bonding of all exterior conductive surfaces to structure and applying anti-static coatings to nonconductive surfaces and bonding these to structure. P-static design currently treats the entire aircraft as worst-case charging areas or relies on qualitative arguments to relieve requirements in low threat areas. For example, qualitative arguments may be based on how a particular aircraft design fared in practice, then assuming that the design guidelines transfer to other aircraft types. These qualitative arguments generate conservative assessments. However, public domain sources indicate that p-static aircraft charging is not uniform, as it depends on the number of particles impacting the aircraft. Treating the entire aircraft as worst case leads to overly conservative guidelines and increased cost.

The p-static guidelines extend to the e use of unbonded conductive materials (i.e., isolated electrical conductors) and dielectric materials on aircraft surfaces. Such materials include speed tape and decals commonly used in-service. In general, external surfaces of the aircraft are to be bonded to each other and then connected to the structure of the aircraft so that any accumulated charge dissipates. As such, speed tape and decal placement are limited, even if the placement of the unbonded conductor is in a low charging part of the aircraft surface.

“Speed tape,” a common name used in the aviation industry, is a dead soft aluminum foil tape with a polymer-based pressure sensitive adhesive that is used to cover damaged surface finishes to prevent ultraviolet (UV) degradation of the exposed unpainted surface. Examples include speed tape with.mil thick aluminum foil with 1.3 mil adhesive (3M425) or 3 mil thick aluminum foil with 2 mil of acrylic adhesive (Permacel P11). Installation includes applying the tape to a damaged surface.

Speed tape thus comprises an electrically isolated conductor due to dielectric properties of the pressure sensitive adhesive. Adhesives used on speed tape are generally polymer acrylic based which have a bulk resistivity much greater than the resistivity that is considered static dissipative. When exposed to triboelectric (e.g., frictional) charging, such as ice particles impinging on airplane surfaces, electrical charge accumulates on the exposed foil until the voltage between the aluminum foil and structure and/or at foil/foil interfaces exceeds the standoff capability of the adhesive layer. This causes periodic uncontrolled discharge at the edges of the tape and through the adhesive. These periodic discharges are radio frequency (RF) sources that propagate RF energy. Such RF energy has the potential to interfere with communication and navigation systems. Interference is dependent on the magnitude and spectra of the RF source received by the system and sensitivity of the system.

As an example,depicts an aircraftwith an aircraft surface. Although primarily described with regards to commercial airliners, “aircraft” as used herein may describe airplanes, non-traditional aircraft, such as fixed wing aircraft and/or vertical take-off and landing aircraft, autonomous and semi-autonomous aircraft, as well as spacecraft, satellites, etc. Aircraftis shown flying through suspended atmospheric particles. Speed tape patches are shown on aircraft surfaceat,, and. A decalis also positioned on aircraft surface. Static dischargersandare also positioned on aircraft surface. Typical speed tape repair application areas are on the nose aft of the landing gear and on the forward wing to body panels. Typical application areas rarely exceed 2 square feet. However, some aircraft are particularly prone to upper wing skin damage, resulting in, much more speed tape being used. Placement of speed tape repairs of total area in specific impingement regions of the nose, wing and empennage on the flight test airplane provides a basis for assessing impact on airplane communications.

The potential for p-static interference on communication and navigation systems is dependent on the charging environment and discharge location, magnitude, and frequency. Certain aircraft are prone to excessive in-service paint damage, and thus rely heavily on speed tape repairs to stay in commission until repainting can occur. Excessive speed tape in high charging areas thus has the potential of generating RF emissions that could affect communication and navigation performance within inclement weather.

The methodology disclosed herein provides aircraft design guidance based on p-static zoning, which takes into account that different areas of an aircraft have different p-static charging threat levels. These different zones are arrived upon via a process that comprises defining a particle impingement profile with a computational fluid dynamics (CFD) model. The CFD model may be validated with a charging profile in p-static flight testing. For example, charge sensors may be positioned on the aircraft surface to gauge charging rates at specific locations of the aircraft surface. The model is then discretized to create p-static zones, with zones having more severe charging profiles having more stringent design guidelines and zones with less severe charging profiles having less stringent design guidelines. This, in turn, reduces risk in p-static mitigation efforts and informs design guidelines for outer mold line materials such as coatings, tapes, and decals, leading to cost savings and cost avoidance in aircraft maintenance. Further, this may improve vendor satisfaction, as airline carriers can use larger decals in certain areas with more variety in materials.

shows a flow diagram for an example methodfor discretizing an aircraft surface into p-static zones. Methodmay be performed in conjunction with one or more computing devices. An example computing device is described herein and with regard to. Methodmay be performed in conjunction with one or more aircraft.

At, methodcomprises generating a particle impingement model for an aircraft surface using computational fluid dynamics (CFD) modelling. The CFD model is a calculation that can be performed with regard to icing impingement and buildup on the nacelle lips (e.g., part of the wing repair area with cumulative area applicable to both wing and nacelles.) or wings. The CFD model may be considered a ballistic model (e.g., Navier-Stokes based model derivation) where particles of differing sizes may be put into analysis to determine where the impingement locations are on the aircraft surface. The analysis may account for wind speed and other atmospheric conditions, but may be considered static e.g., does not consider motion. In some examples, other ballistic models may be employed. Computational fluid dynamic (CFD) analysis of particle impingement on the airplane may be used to determine placement of repair patches.

show one example of CFD analysis according to the present disclosure. The analysis is an impingement static model with select particle sizes. The model used a “bouncing off” representation of particle impingement with the airplane at cruise altitude (39,000 ft), speed and cruise angle of attack (AOA). Three simulations were run, with particle sizes 50 μm (), 200 μm () and 1000 μm (). Unspeckled portions of the depicted aircraft have a negligible value for β, used as a measure of particle impingement. Heavier speckled portions of the depicted aircraft have progressively larger values for β.

The 50 μm () and 200 μm () models simulated min/max range of precipitation particles. The 1000 μm particle model () simulates an upper bound case. The output of this model is the value β that describes the particle impingement, where β=0 indicates no particle impingement and β is at its maximum where particle impingement is at a maximum. β is defined as the local droplet flux rate at the body surface normalized to the freestream flux rate. β is a function of droplet size and can be expressed as the local impingement efficiency for any point on the body surface.

shows a head-on depictionof an aircraft.shows an undersideof an aircraft wing. Particle impingement (for particle sizes of 50 μm) is notable in aircraft surface regions including on the nose of the aircraft, on the leading edge of the wings, on the engine housings, on the leading edge of the stabilizers, and on the leading edge of the fin.

shows a head-on depictionof an aircraft.shows an undersideof an aircraft wing. Particle impingement regions (for particle sizes of 200 μm) with a positive value for β are similar to those particle impingement regions shown for. However, values for β in those regions are increased over those for 50 μm particles, and the regions have larger surface area as compared to those for 50 μm particles.

shows a head-on depictionof an aircraft.shows an undersideof an aircraft wing. Particle impingement regions (for particle sizes of 1000 μm) with a positive value for β, are similar to those particle impingement regions shown for. However, values for β in those regions are increased over those for either 50 or 200 μm particles, and the regions have larger surface area as compared to those for either 50 or 200 μm particles.

One characteristic that the particle impingement simulation results indicate is that β regions from the 1000 μm particle have the largest impingement area as compared to smaller sized particles, where the impingement area is relatively smaller due to the aerodynamic interaction with the particles. Larger, heavier particles (e.g., 1000 μm) are less affected by the airstream and therefore impinge farther aft than smaller particles and over a larger portion of the aircraft surface as compared to smaller particles. Notably, the higher β regions do not shift aft as much compared to the lower β regions for larger particles.

Returning to, optionally, at, methodcomprises generating a charging profile for the aircraft surface based on flight test data from charging patches positioned on the aircraft surface. Charging patches can be positioned on the aircraft surface based on the particle impingement model. The charging profile for the aircraft surface can be further based on flight test data from instrumented static dischargers positioned on the aircraft surface.

The charging patches provide a charging profile of the airplane in flight. Charging patches may be placed strategically at different angle of attack regions around different parts of the aircraft surface to inform the charging rate for an aspect angle of the exposed aircraft surface. A charging profile may be generated including a charging current density at a diverse set of points around the aircraft surface.

A charging patch at the most forward part of the wing may correlate to the highest charging area of the aircraft surface, while a charging patch on the fuselage might not experience as much charging. Data can be collected from regions of the aircraft surface that span the range from lowest particle impingement to most particle impingement. Overall, the charging patches may span a large range of different particle impingement βvalues (e.g., 0.01 to 1) and different charging profiles across the aircraft, allowing for validation of the entire CFD model.

The function of each charging patch is to provide particle impingement charging rate data with respect to airplane speed and effective charging area with respect to the angle of attack (AOA) of the impinging particles. The charging patch simply measures current from triboelectric charge separation from impinging suspended precipitation particles. Current magnitude is a function of particle type and size, particle charge, particle density, area of charging patch, patch AOA, patch material, the aero-boundary layer, and aircraft speed. The effective area of the charging patch correlates with the interaction of particles based on the projected frontal area of the patch. Essentially, the higher the AOA, with respect to the AOA of the airplane, the lower the effective charging area.

Impinging particles generate relatively small charge exchanges so measurements are generally amplified. Such patches themselves may be-by way of non-limiting example-an electrically isolated strip of 3M425 speed tape (e.g., measuring 1″ wide by 4″) spanwise, positioned on top of a layer of urethane tape. A corona guard ring may be positioned around the patch which helps reduce brush discharges from the urethane tape and from surrounding composite surfaces. The patch may be attached to a resistor in parallel with an amplifier module. The wiring may include an RF shield attached to the corona guard ring and at the pressure vessel penetration. Such a shield reduces electromagnetic interference (EMI) and provides a ground for the corona guard. The transient suppression device clamps conducted and induced voltages into the pressure vessel in the event of a lightning attachment to or near the patch. The transient suppression device also clamps conducted and induced voltages at the pressure vessel in the event of a lightning attachment to or near the Instrumented Static Dischargers (ISDs).

Patch placement may be determined at least in part based on the β values for the aircraft surface generated via CFD analysis. In one example, individual charging patches were applied to a fixed leading-edge surface with patches positioned at 0°-10°, 20°-30°, 45°-50°, 65°-70° and 85° with respect to the forward apex of the leading-edge surface. It has been shown that ice crystals to snow particles have an impingement area between 10° to 30° with larger particles such as water droplets including an impingement angle up to 40°.

The ISDs are trailing type static dischargers that have been modified to electrically isolate the static wick (resistive rod) from the discharger base (housing) and attach a wire to the isolated static wick. This allows the wiring to be attached to instrumentation, allowing measurement of discharge current at the mounted location of each ISD. Similar to the charging patches, the transient suppression device clamps conducted and induced voltages at the pressure vessel in the event of a lightning attachment to or near the ISDs and do not affect the measured discharge current. ISDs measure the discharge rate of the airplane. ISDs are more susceptible to exogenous charging, whereas the charging patches respond more to frictional charging. This combination of charge patches and ISDs helps to separate exogenous charging from triboelectric charging. The measurements from the most outboard ISDs are used to establish the most severe airplane charging environment during operation of communication and navigation systems. They also help delineate triboelectric charging conditions from exogenous electrification of the airplane. The outputs of the other ISDs aid in understanding the airplane charging environment and verification of triboelectric charging of the airplane. They also provide insight into charge redistribution on the airplane for analysis.

Charging patches may be positioned at specific angles with respect to 0° pitch (e.g., parallel to horizon not including the angle of attack of the airplane) on the leading edge of the left wing and vertical stabilizer. These patches establish the triboelectric charging environment with respect to angle of attack (AOA).

shows an example aircraftwith charging patches and ISDs mounted on the aircraft surface. Four regions of the airplane were selected for patch placement including the fuselage, two regions of left wing, and fin. The wing tip charging patches were installed on left leading edge of the wing. Instrumentation also includes instrumented static dischargers (ISDs) mounted on left wingand left horizontal tip. The AOA for the patches were selected from past data and particle impingement regions of various particle sizes. A single patch(fuselage 1) was placed on fuselage, with an approximate AOA of 90°. A four-patch cluster (left wing patch 1, left wing patch 2, left wing patch 3, left wing patch 4) was placed on the leading edge of the wing tip of left wing. Two patches (left wing patch 5and left-wing patch 6) were placed midspan of left wingat 85° AOA. A four-patch cluster (vertical fin patch 1, vertical fin patch 2, vertical fin patch 3, vertical fin patch 4) was placed on the leading edge of fin. ISDs were mounted on left wing(ISDsand) and the left horizontal tip(ISD). The configuration shown inis merely exemplary, and more or fewer charging patches and ISDs may be mounted on the aircraft surface in other examples. Locations for charging patches and ISDs may also vary.

Based on past data, the most forward two patches (left wing patch 1and left-wing patch 2) would capture the highest amount of charge from a range of precipitation particle sizes, with left wing patch 3and left-wing patch 4being much lower. The midspan wing (and) and fuselage () patches, located in a β region less than 0.1, are intended to capture charging during severe charging conditions where the effective charging area is essentially zero.

Table 1 shows the location for each charging patch and its associated B value.

Table 2 shows total charge accumulated on charging patches for the duration of the flight test.

The charging profile acquired via the charging patches may be used to validate (and/or adjust) the particle impingement model. For example, uncertainty bounds for a particle impingement model may be adjusted based on the charging profile.is an example plotthat shows the total charge accumulated (ρ) on each patch during the flight test against β(β at the center of specific charging patch) predicted by the 1000 μm particle impingement computational fluid dynamics (CFD) model (). The correlation coefficient between the ρand β is 0.97, indicating a very strong correlation. The largest source of uncertainty in βcomes from estimating the charge patch locations on the particle impingement model that uses in-flight geometry from simple jig geometry callouts in the installation worksheet. This uncertainty is reflected in the variability in βwith respect to ρ. Due to the strong correlation between ρand β, the result can be fit with the linear regression ρ=Cβ+Cwhich gives c=3.3±0.5 cmand c=0.0±0.3 cm. Note that because ρis the charge accumulated for the entire duration of the flight test, and depends on the flight conditions, the coefficient, cis dependent on the specific flight or segment of flight that is analyzed. However, cshould always be approximately 0 cm. This fit results in R=0.95, or 95% of the ρresult is explained by this linear model.shows this linear fit as the solid line with 95% confidence intervals (simultaneous observation bounds) as the dashed lines.

This result validates the predictive ability of β in that it is proportional to the total accumulated charge. Additionally, this result validates the use of β for predicting the charging current density profile because the accumulated charge density is linearly related to the average charging current densityover duration Δt by ρ=∫Jdt=Δt. This β prediction and charge patch data can therefore be used to predict the charging current density across the entire aircraft during each flight test condition, including at all unbonded conductor locations.

Returning to, at, methodcomprises discretizing the particle impingement model to generate two or more p-static zones. As used herein, discretization is used to indicate dividing the aircraft surface into two or more distinct zones that are treated individually, even if the zone has a range of particle impingement values. Optionally, at, methodcomprises discretizing the particle impingement model based on the charging profile. Discretization may be based on a correlation of an output of an impingement model with surface charge accumulation. P-static zones may be based on a combination of the two data sets and might not directly correlate with either model in totality.

P-static zones may be characterized by a charge current density severity, and wherein a stringency of p-static design guidelines for a given p-static zone correlates to the charge current density severity for the given p-static zone. Current density profiles for an aircraft surface may be generated from a combination of the validated particle impingement model and the charging patch data.

schematically shows p-static zones for an aircraft fin, andschematically shows p-static zones for an aircraft airfoil.schematically shows an example aircraft fin. The surface of finis shown divided into three p-static zones—zone 1, zone 2, and zone 3. A cross section of finis shown at. While three zones are depicted, in other examples there may be as few as two zones or more than three zones.

Zone 1may be considered a high β/high charging current density zone. Zone 1roughly correlates to the edge of the leading-edge skin and the forward torque box skin. Zone 2may be considered a medium β/medium charging current density zone. Zone 2roughly correlates to the edge of the forward torque box skin and the main torque box skin. Zone 3may be considered a low or negligible β/low charging current density zone. For reference, cross sectionindicates aux sparand the front spar of the main torque box.

schematically shows an example aircraft wing. The surface of wingis shown divided into three p-static zones—zone 1, zone 2, and zone 3. A cross section of wingis shown at.

Zone 1may be considered a high β/high charging current density zone. Zone 1roughly correlates to the edge of the leading-edge skin and the aux box skin. Zone 2may be considered a medium β/medium charging current density zone. Zone 2roughly correlates to the edge of the aux box skin and the multispar box. Zone 3may be considered a low or negligible β/low charging current density zone. For reference, cross sectionindicates aux sparand the front spar of the multispar box.

Returning to, at, methodcomprises establishing p-static design guidelines on a per p-static zone basis. A p-static design guideline can comprise a surface area of unbonded conductive or dielectric material within the respective p-static zone. The unbonded conductive material may comprise speed tape. The dielectric material may comprise decals.

In zones with low β (and/or low charging current density), the amount of unbonded conductive material and/or dielectric material may be increased above FAA guidelines. In some examples, there may be zones with little to no restrictions on unbonded conductive material area. Zones with high β (and/or high charging current density) may allow little or no unbonded conductive material. Zones with intermediate β (and/or intermediate charging current density) may have restrictions that allow a certain area of unbonded conductive material. The p-static zones may also be based on the location of antennas relative to location on the aircraft surface. For example, an aircraft surface portion with an intermediate β that is located close to an antenna may be clustered with zones comprising surface portions with high β values. As an example, the lowest charging current observed during a test condition was 2 μA on the horizontal stabilizer and the highest was 340 μA on the nose. The relative uncertainty of these currents is 50% and 80%. However, even with high discharge rates, the coupled noise to the antennas may be sufficiently low as to not create a p-static interference issue with any of the communication or navigation systems. The tested repair patch configuration may thus bound speed tape installations of the same area in lower β regions. However, because the charging current density is lower, greater speed tape area than what is tested is also acceptable.

For each unbonded conductive patch or decal, it is known from modeling and charge patch data the exact current density is impinging or charging each one of those repair patches. The current density onto the repair patches creates the noise that then couples to the antennas. For charging patches and areas with lower current density, there would need to be more repair patches to attain an equivalent threat level. For example, repair patches may be positioned at or close to β=0.3 regions and aft from the 1000 μm particle model. Higher β regions are generally either on areas of the airplane that prohibit foil speed tape repairs (e.g., nose and radome) or are in areas of bare metal (e.g., engine inlet lip, slats, and empennage bullnose) that do not typically need speed tape repairs.

Returning to, at, methodcomprises applying one or more of unbonded conductive material and dielectric material to the aircraft surface based on the p-static design guidelines. For example, a computing system may indicate application of unbonded conductive material and/or dielectric material to the aircraft surface to a user trained to apply the unbonded conductive material and/or dielectric material to the aircraft surface.

illustrates unbonded conductive material and dielectric material applied to an aircraft fin on a per p-static zone basis.schematically shows an example aircraft fin. The surface of finis shown divided into three p-static zones-zone 1, zone 2, and zone 3, however in other examples there may be as few as two zones or more than three zones. Zone 1is a high charge current density zone that restricts any unbonded conductive material and dielectric material. Zone 2is an intermediate charge current density zone that allows a certain area of unbonded conductive material and/or dielectric material. Here, speed tape is shown applied at,, and. Zone 3is a low charge current density zone that allows unlimited unbonded conductive material and/or dielectric material, such as decal.

An acceptable application area for unbonded conductive material and/or dielectric material may be established per p-static zone based on the β value and/or charge current density for each zone. There exists a voltage threshold, Vthat initiates electrostatic discharge of the average repair patch. This electrostatic discharge potentially couples to an antenna, creating p-static interference. The charge required to achieve this breakdown threshold for a single speed tape patch is Q=C V, where C is the capacitance of the patch. The discharge rate for the patch is proportional to the charging current density and patch area and inversely proportional to the patch capacitance and voltage threshold.

Another relevant quantity when comparing two speed tape installations is the amount of energy involved in the discharge. This energy can be calculated by assuming the entire stored charge in the patch is involved in the discharge, which allows for treating the patch as discharging capacitor. The p-static threat from an isolated conductor like a repair patch (conductor n) bounds the threat of another patch (conductor m) when: 1. The number of discharges per second from conductor n is greater than that from conductor m and; 2. The discharge energy from conductor n is greater than that from conductor m. The number of discharges and discharge energy can be combined through analyzing the average power created from the discharging conductor over the duration of the test condition.

For patch clusters, the total radiated power from a patch cluster is the sum of average radiated power from each individual patch, However, the power that is available to couple to a given antenna depends on the repair patch's location on the aircraft relative to the antenna and the frequency band of the antenna. Method of moments (MoMs) simulations can be used to characterize this sensitivity.